According to one basic solution of the above problem it has been proposed in the prior art to connect to the liquid inlet of the chromatograph proper a feeding line provided with check-valves and associated to two single-acting cylinder and piston pumping units connected to the feeding line respectively at a first connecting point and a second connecting point located down-stream from said first connecting point, with reference to the direction of the flow of liquid to be analyzed. The above-mentioned check-valves interposed in the feeding line, as well as the driving and/or control means of said pumping units, are arranged in such a manner that when one of said pumping units performs its suction stroke, drawing liquid from the feeding line into the cylinder of said unit, the said pumping unit is only connected by the feeding line to the liquid source, but is disconnected from the chromatograph, whereas at the same time the other pumping unit, performing its discharge stroke, is connected to the chromatograph and disconnected from the liquid source, so as to provide the required liquid flow toward the chromatograph while the first-mentioned pumping unit is in the process of taking in liquid by suction.
It should be noted that for obvious technical reasons it is desirable that the intake or suction stroke of every pumping unit be performed as rapidly as possible or, in other words, take up the smallest possible fraction of the total time required for performing the complete pumping cycle constituted by the suction stroke and the discharge stroke.
Supposing that in a given installation of the type considered herein-before the first pumping unit (or upstream unit), which is connected to the first connecting point, has a capacity V.sub.1 of 300 cm.sup.3, its suction stroke time t.sup.A.sbsb.1 being 10 seconds and its discharge stroke time t.sub.R.sbsb.1 being 30 seconds, while the second pumping unit (or downstream unit), which is connected to the second connecting point, has to take in, during the discharge stroke of the first unit, a predetermined proportion of the 300 cm.sup.3 discharged from the first unit, which portion of liquid will subsequently be discharged by said second (or downstream) unit during the suction stroke of the first unit, so as to maintain the flow rate of the liquid delivered to the chromatograph at a constant value.
On the base of these data, the first pumping unit will provide an output of liquid during its discharge stroke which equals EQU (V.sub.1 /t.sub.R.sbsb.1)=(300 cm.sup.3 /30 s)
With a view to feeding the chromatograph at a constant flow rate, as required, the second pumping unit must then have a capacity of 75 cm.sup.3, its discharge stroke time must be 10 seconds and its suction stroke time 30 seconds. Under these conditions, the second unit will take in 75 cm.sup.3 from the 300 cm.sup.3 output of the first unit, leaving 225 cm.sup.3 to be delivered to the chromatograph during the 30 seconds of the discharge stroke time of said first unit, and will deliver said 75 cm.sup.3 to the chromatograph during the 10 seconds of the suction stroke time of the first unit.
In other words, the chromatograph will be fed with a constant flow rate of 75 cm.sup.3 per 10 seconds, provided, of course, that the pistons of the pumping units are driven in accordance with a linear characteristic. Obviously, if the respective stroke lengths of the two pistons are equal, their cross-sectional areas must be proportional to the respective output quantities of the units (in the present example, proportional to 300/75). The pistons will be actuated in a mutually reversed manner, i.e. one unit performing its intake stroke while the other performs its output stroke, which is easily achieved by well known common driving means, such as a shaft provided with conveniently shaped and positioned actuating cams.
The above conditions can be expressed by the general equation: EQU (V.sub.2 /V.sub.1)=t.sub.A.sbsb.1 /(t.sub.A.sbsb.1 +t.sub.R.sbsb.1)
wherein:
V.sub.1 =capacity of the first (upstream) pumping unit; PA1 V.sub.2 =capacity of the second (downstream) pumping unit; PA1 t.sub.A.sbsb.1 =suction stroke time of the first pumping unit; PA1 t.sub.R.sbsb.1 =discharge stroke time of the first pumping unit.
Indeed, the ratio of the complete duration of the operating cycle (suction plus discharge) of the first pumping unit (upstream unit) to the duration of its suction stroke obviously must be equal to the ratio of the capacity of said first (upstream) unit to the capacity of the second (downstream) unit with a view to allowing said second unit to feed on the output of the first unit, during the discharge stroke of the latter, with an amount of liquid sufficient to subsequently feed the chromatograph at a constant flow rate during the suction stroke of the first unit.
Such known arrangement thus theoretically allows a constant rate of flow towards the chromatograph to be achieved. However, in practice, it has been observed that the suction effect due to the operation of the first unit in the feeding line upstream from the first (or upstream) connecting point being interrupted during the suction stroke of the second unit (which takes in liquid not directly provided by the upstream portion of the feeding line, but by the liquid output provided by the first unit during the discharge stroke thereof), that consequently there is a considerable hazard of generating undesirable phenomena such as cavitation and the like, which would substantially impair the imperatively required constant flow rate of the liquid delivered to the chromatograph. Such risk increases as the flow rate of liquid delivered to the chromatograph is increased. It also increases with the frequency of the strokes of the pumping unit cylinders.
The present invention is aimed in particular at overcoming this drawback of the conventional systems comprising two pumping units.
It is thus one main object of the invention to provide a pumping system which ensures achievement of a strictly constant feeding flow rate of a chromatograph or the like, whichever may be the absolute respective values of the required flow rate and the respective values of the suction and discharge stroke times of the pistons in the pumping units.